The Science: The Earth and Moon

Eclipses, both solar and lunar, are a
result of the movements of the Sun, Moon, and Earth. This page takes
a very brief look at our local bit of the solar system, and explains
the basics of how the Earth and Moon move relative to the Sun.

The Sun and its Companions

The Sun is the centre of our
solar system, and by far the dominant
object in it. As well as providing the light and heat which power
all life on Earth, it is by far the most massive object in the solar system:
more than 300,000 times heavier than the Earth, and over 700 times heavier
than all the planets put together. This huge mass
acts as the anchor for the whole system: all of the other objects in the
solar system — planets, asteroids, moons, etc. — are in orbit around
the Sun, either directly, or indirectly as moons of other objects.

The main other objects in the solar system are the eight planets, each of
which circles the Sun in its own orbit. Our own Earth is the third planet
out from the Sun. Most of the planets have moons in orbit about them,
and Earth is no exception; our Moon orbits around
us once per month, and so gets carried along with us in our journey around
the Sun.

Our Moon is fairly unremarkable, except for its size; it is one of the
larger moons in the solar system. The main reason it looks so big to
us, though, is simply that it is very close; the Moon is just
384 thousand km away. By comparison, the Sun is 150 million km away.

The Earth and Moon

Although the whole solar system is fascinating, the other planets don't
play a role in eclipses. For our purposes, we're interested in the Sun,
the Earth, and the Moon, and how they relate to each other.

This diagram shows the Sun, Earth, and Moon, and how their orbits work.
The scale is hugely exaggerated, as a scale diagram would be hideously
impractical (think about the sizes and distances above, and you'll see
what I mean):

In this diagram, the Sun is shown as the orange ball. The Sun is the centre
of the solar system, so when discussing the structure of the solar system,
it's appropriate to think of the Sun as sitting still (though it does
actually move within our galaxy).

Orbiting the Sun, I have shown the Earth, with its orbit shown as a blue
circle — the other planets are not shown. The view is from
above the Earth's north pole, and from this
point of view, the Earth goes round the Sun anti-clockwise, as shown by the
arrow on the orbit. The direction of the Earth's rotation is shown by
the curved blue arrow beside the Earth; again, as seen from above the
north pole, this is anti-clockwise.

The Moon is the only natural object orbiting the Earth (there are thousands
of man-made satellites). I have shown it here as the white ball, and its
orbit as the white line; its direction of rotation is shown as the small
white arrow. Again, you can see that the Moon's orbit, and its direction of
rotation, are anti-clockwise as seen from above the north pole.

So — bearing in mind the terribly distorted scale — this is how our
Earth and Moon relate to each other, and to the Sun; and it is the motions
shown in this diagram that cause eclipses. But before we get into that,
let's look at how this works in practice.

Day and Night

Given the motion of the Earth as described above, we can see how night
and day work. Here's a picture that shows how the Earth is lit up
by the Sun:

As you can see, at any given time, half the Earth is illuminated by the
Sun — that is the half of the Earth on which people can see
the Sun, and this is the day side. The other half of the Earth receives
no direct light from the Sun, and hence is in darkness; looking at it from
our Earthly point of view, the Sun isn't visible because it's round behind
the other side of the Earth. This is the night side.

Day and night aren't permanent, of course; we're used to the Sun rising,
moving across the sky, then setting, then rising again. This happens because
the Earth is rotating, as shown by the arrow in the diagram above, so that
over the course of 24 hours — which is how long it takes the Earth to do one
complete rotation — every part of the Earth sees a day and a night. It
is the rotation of the Earth that makes the Sun appear to move across the
sky, when it is actually sitting still.

Somebody standing on the Earth would be on the side facing the Sun at noon.
The rotation of the Earth would carry them anti-clockwise towards the point
marked sunset; from their point of view, the Sun would appear to be
going the other way, and to be getting lower in the sky. When our observer
reached the sunset point, the Sun would be exactly on the western horizon from
their point of view; then they would move around to the night side.

At midnight
they would be on the side of the Earth opposite the Sun; then they would move
on towards the point marked sunrise, where they would see the Sun
appear on the opposite (eastern) horizon. As the Earth continues rotating,
they would see the Sun climb in the sky until they were once again directly
in line with it at noon.

Phases of the Moon

The Moon is — by far — the brightest object in the night sky,
far outshining all the stars and other planets. So what makes it glow?
Well, in fact, it doesn't "glow" at all — like all the other planets,
the Moon is only visible because it reflects light from
our Sun, the only
large-scale source of light in our solar system. (The stars, of course,
are Suns in their own right, and in their own solar systems). The Moon
is very bright simply because it is pretty big, and very close to us
compared to the other planets, and so it reflects a lot of sunlight our way.

Every month, though, something happens — the Moon shrinks, first to a
half moon, then a crescent, then vanishes entirely at the New Moon. Then,
it starts growing again — from crescent, to half, and then to Full Moon.
These are called the phases of the Moon — but what makes them happen?

It's a not uncommon misconception that the Moon goes through these phases
because of the shadow of the Earth.
This isn't true, though — if you look at the diagram above, you will see that
the Moon couldn't be in the Earth's shadow for more than a small portion
of its orbit at most. In reality, if we were to look at a properly scaled
diagram, we would see that the Earth is even smaller in proportion to the Moon's
orbit, and so the Moon wouldn't be in the Earth's shadow for more than a few
hours a month. Since the phases of the Moon last for a whole month, something
else must be causing them.

The answer is that the Moon is only half-lit; and we're seeing different
angles of the half-lit Moon as it orbits around the Earth. Just like the
Earth, only the side of the Moon facing the Sun is lit by it; the rest is
in darkness. When we look at the Moon, depending on the angle it is at,
we may see the lit side, or the unlit side, or part of the lit side and
part of the unlit side; and this is what causes phases.

The Moon goes through a complete cycle of phases — from New to Full
and back to New — in one orbit around the Earth; this is a lunar
month, which is 29.5 days on average. (Getting a lot more technical,
this is actually a synodic month). This picture shows how the Moon is illuminated by the Sun at
different times during a lunar month as the Moon orbits the Earth:

New Moon

Starting at the New Moon, we can see that the Moon is on the same
side of the Earth as the Sun. This means two things:

the night side of the Earth is facing the wrong way to see the Moon.
Because the Moon and Sun are in the same part of the sky as seen from Earth,
if it's night — ie. the Sun is down — then the Moon must be down too.
In other words, the Moon is only up (above the horizon) in daytime. It rises
at Sunrise, and sets at Sunset.

The lit side of the Moon is turned away from Earth; the dark side is
towards us. There is very little illumination on the dark side — there is
the reflected light from Earth, but that's much weaker than direct Sunlight —
so the Moon is very dim.

The combination of these things — the Moon being dark, and only up
in daytime — makes the New Moon pretty much impossible to see.

Waxing Crescent Moon

As the Moon moves anti-clockwise in its orbit, it moves away from the
line between the Earth and Sun. As it does so, it is no longer lined up
perfectly to turn its dark side towards us, and a bit of the light side
becomes visible; this appears to us as a crescent Moon. Look at the
diagram above, and you should be able to see how Earth's view of the
crescent Moon is mostly dark, but with a sliver of light. Because the Moon
is growing from the New Moon, this is called the waxing crescent
("waxing" is an old word for "growing").

You'll also notice that while the crescent Moon can be seen mostly
by people on the daylight side of the Earth, it's also visible from the
night side just after the sunset point. This means that the crescent Moon
can be seen from mid-morning, when it rises, through to just after sunset,
when it sets, if the sky is clear enough. With the Moon being close to the Sun,
it might be drowned out by the Sun's light in daytime, particularly if it's
a little hazy.

At night, though, the crescent Moon can be a beautiful sight; and if you
take the time to have a look at it, you may see something even more beautiful.
Because the dark side of the Moon is mostly facing the lit side of the Earth, it
actually gets illuminated by reflected Earthlight. Although this is much
dimmer than the Sunlight which is lighting up the crescent, you can quite
often see the whole Moon cradled in the arms of the crescent Moon, lit up
by Earthlight. When you see this, you are looking at light that has
travelled from the Sun, then bounced of the Earth, then bounced again
off the Moon, and travelled back to the Earth, to be seen by you. This
is something to look out for around dusk in the days after the New Moon.

First Quarter

As the Moon continues anti-clockwise in its orbit, it moves around to the
point marked First Quarter. Now, the term "quarter" here refers to
the lunar month — we are a quarter of the way in to the lunar month at this
stage. However, looking at the diagram, you can see that people on the
Earth looking "down" at the First Quarter Moon see part of the lit side,
and part of the unlit side; in fact, what they see is a half Moon.

As you can see, the First Quarter Moon can be seen by some people
on the daylight side of the Earth, as well as by some on the night side —
in fact, the First Quarter Moon can be seen from noon, when it rises,
through to midnight, when it sets, if the sky is clear enough.

Waxing Gibbous Moon

As the Moon moves on, it keeps growing, and becomes a gibbous Moon,
which is a half Moon with a bulge — part way between a half Moon and
Full Moon.

Full Moon

At the next stage, we can see that the Moon is on the opposite
side of the Earth from the Sun. This means two things: the dark side of the
Earth — which is to say, the part which is currently in night-time — is
the part which can see the Moon; and it sees the whole lit side of the Moon.
This is a Full Moon, which rises at Sunset, and sets at Sunrise.
We are now half way through a lunar month.

Waning Gibbous Moon

The second half of the lunar Month is the reverse of the first half.
As the Moon moves past Full, it starts to shrink, becoming a gibbous Moon;
this time, the waning gibbous moon, as it's shrinking ("waning").
This Moon is visible from early night-time to mid-morning, if the sky
is clear enough to see it in daytime.

Last Quarter

The Moon again reaches the half-Moon stage at Last Quarter (ie. the
last quarter of the lunar month). At this stage, we again see half
the Moon lit, and again the Moon is visible from both the day and night
sides of the Earth; looking at the diagram above, you'll see that the
Moon is visible from the midnight point, to Sunrise, through to noon.

Waning Crescent Moon

After the Last Quarter, the Moon diminishes once again to a crescent,
now visible from just before dawn through to afternoon; this is the
waning crescent moon.

New Moon

Finally, the crescent Moon shrinks to nothing at the next New Moon,
and the cycle starts all over again.

And Eclipses?

So what about eclipses? And what about the shadows of the Earth and Moon
that we mentioned earlier?

Well, as we'll see, these subjects are closely related. Now that you
know about the basic mechanics of the Earth and Moon, we can go on to
look at how eclipses work.

Now go on and read about how solar and
lunar eclipses work. If you're interested in
learning more about the cycles of the Moon, there's a section on
lunar months which goes into more
detail.